1. Technical Field
The present invention generally relates to a method and apparatus for writing data to and/or reading data from a storage surface via a tip.
2. Discussion of Related Art
An example of a such a storage device is described in “The “Millipede”—More than one thousand tips for future AFM data storage”, P. Vettiger et al., IBM Journal of Research and Development. Vol. 44 No. 3, May 2000. As described therein, this device comprises a two dimensional array of cantilever sensors fabricated on a silicon substrate. Each cantilever is attached at one end to the substrate. The other end of each cantilever carries a resistive heater element and an outward facing tip. Each cantilever is addressable via row and column conductors. The row and column conductors permit selective passage of electrical current through each cantilever to heat the heating element thereon.
In both reading and writing operations, the tips of the array are brought into contact with and moved relative to a storage medium comprising a polymer layer such as a layer of polymethylmethacrylate (PMMA) coating a plane substrate.
Data is written to the storage medium by a combination of applying a local force to the polymer layer via each tip and selectively heating each tip via application of data signals through the corresponding row and column conductors to a level sufficient to locally deform the polymer layer, thereby causing the tip to leave an indentation or pit in the surface of polymer layer. Conventionally, the local force is applied by mechanically biassing the tip against the polymer layer. The storage medium can be thermally erased and then rewritten multiple times. To erase the storage medium, the polymer layer is heated to a level sufficient reflow the polymer layer thereby removing all indentations recorded in the storage medium.
Each heating element also provides a thermal read back sensor because it has a resistance which is dependent on temperature. For data reading operations, a heating signal is applied sequentially to each row in the array. The heating signal heats heating elements in the selected row, but now to a temperature which is insufficient to deform the polymer layer. The thermal conductance between the heating elements and the storage medium varies according to distance between the heating elements and the storage medium. When the tips move into bit indentations as the array is scanned across the storage medium, the distances between the heating elements and the storage medium reduce. The medium between the heating elements and the storage medium transfers heat between the heating elements and the storage medium. Heat transfer between each heating element and the storage medium becomes more efficient when the associated tip moves in an indentation. The temperature and therefore the resistance of the heating element therefore reduces. Changes in temperature of the continuously heated heating elements of each row can be monitored in parallel, thereby facilitating detection of recorded bits. Conventionally, relatively long tips have been employed in the interests of achieving acceptable signal to noise ratios. However, such long tips are relatively delicate and difficult to manufacture. Also, in conventional devices, relatively soft polymers are employed in the interests of ease of writing. However, a problem associated with such materials in that deformations therein are relatively sensitive to temperature changes. Specifically, the deformations can be removed by changes in environmental temperature, resulting in a corresponding loss of data. Additionally, as the array is scanned across the surface, the tips wear against the polymer material.